The scheme tzs came up with provides questionable security, which was also pointed out by other commenters. It offers no protection against hiding the fact that you are in posession of a known file.
For instance, say you are a whistleblower and you have a stash of documents nobody should know you have. Your opponent, having a copy of those documents, can produce an identical encrypted file. What's more, Dropbox obviously already has a mechanism to look up digests so checking wheter the document is stored with Dropbox or not is probably a matter of milliseconds.
Also, as someone pointed out, deriving the key from the cleartext is probably a very Bad Idea.
The only workable approach I can think of is to encrypt and decrypt data on the client. Any scenario where encryption takes place on the server is suspect.
My personal workaround for this is to use Dropbox to store encrypted filesystem images that I can mount on my machine. This severely limits the usefulness and performance of the service, but has turned out to work reasonably well.
Precisely this. Encfs is particularly good for this if you're on a unix-y environment. Otherwise, TrueCrypt might fit the bill (though concurrent access from several machines can be an issue)
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The scheme tzs came up with provides questionable security, which was also pointed out by other commenters. It offers no protection against hiding the fact that you are in posession of a known file.
For instance, say you are a whistleblower and you have a stash of documents nobody should know you have. Your opponent, having a copy of those documents, can produce an identical encrypted file. What's more, Dropbox obviously already has a mechanism to look up digests so checking wheter the document is stored with Dropbox or not is probably a matter of milliseconds.
Also, as someone pointed out, deriving the key from the cleartext is probably a very Bad Idea.
The only workable approach I can think of is to encrypt and decrypt data on the client. Any scenario where encryption takes place on the server is suspect.
My personal workaround for this is to use Dropbox to store encrypted filesystem images that I can mount on my machine. This severely limits the usefulness and performance of the service, but has turned out to work reasonably well.
Precisely this. Encfs is particularly good for this if you're on a unix-y environment. Otherwise, TrueCrypt might fit the bill (though concurrent access from several machines can be an issue)
If you are a whistleblower storing such sensitive documents on Dropbox, you have other issues to worry about.